1. Field Of The Invention
This invention relates to a variable impedance transformer, and more specifically to a method and means for minimizing an alternating voltage induced in control windings.
2. Background Of The Invention
Saturable reactors, and more specifically variable impedance transformers provide an extremely rugged, substantially maintenance free means to control large amounts of AC power delivered to large lighting loads, heavy duty electric motors and the like. The high secondary AC power levels are controlled by relatively low DC control power levels wherein the DC control power establishes levels of magnetic flux saturation in appropriate cores proportional to the required AC power output level as is well known to those skilled in the art. Offsetting these desirable characteristics are some disadvantages in using these systems. The variable impedance transformer is bulky, heavy, and has a relatively slow response time when compared to other power control systems. A final problem encountered with saturable reactors, and more particularly a variable impedance transformer is the alternating voltage induced in the DC control windings by the magnetic flux within the AC primary windings/ DC control winding common core(s).
The induced alternating voltage in the DC control windings places restrictions on the design and operation of the DC control power source. Designers have attempted to solve this deficiency by installing bulky heat sinks, large semiconductors and resistors in parallel with the control windings. Various resistance-capacitance solutions have been described and some designers have attempted to solve this problem by placing a plurality of opposed DC control windings on the control core such that the induced AC voltages cancel each other. In another system, a plurality of AC primary winding/DC control winding common cores are oriented in a manner such that the magnetic flux of a first core flows in opposition to the magnetic flux of a second core proximate the DC control winding thereby having a substantially canceling effect of the magnetic fluxes thereby minimizing the induced AC voltage in the DC control winding.
U.S. Pat. No. 2,498,475 to John Q. Adams teaches a saturable magnetic core with a core construction possessing a characteristic of constant permeability over a specified range of magnetomotive force. Utilizing a two section core assembly with a DC polarizing coil around a first section of the core assembly such that the algebraic sum of the magnetization curves of the polarized and unpolarized core sections is a straight line.
U.S. Pat. No. 2,586,657 to William J. Holt, Jr. teaches a variable voltage transformer for controlling a load voltage by means of an adjustable DC voltage applied to a DC control winding. The device utilizes a plurality of cores with two primary windings, each of the primary windings is simultaneously wound about a secondary core and a saturable core. A secondary winding is wound about each of the secondary cores and the secondary windings are connected in parallel to the load. The DC control winding is wound about both of the saturable cores for controlling the flux level in each of the saturable cores. A flux is induced in each of the saturable cores which are positioned proximate each other by means of an AC voltage applied to the primary windings. The fluxes are opposite and equal to each other, thereby canceling each other and thereby producing substantially zero or little induced AC voltage in the control winding.
U.S. Pat. No. 2,870,397 to Fred W. Kelley, Jr. teaches an improved saturable core apparatus utilizing three cores with two of the cores being saturable by means of a DC control source and the third core acting as a flux conductor for a primary input and secondary output transformer. Two primary windings are wound about the third core in parallel with opposing diodes or rectifiers placed in the path of the primary windings so that the windings only conduct during alternate half cycles of an AC wave.
U.S. Pat. No. 3,087,108 to Domonic S. Toffolo teaches the efficient transfer of power from a source to a load which can operate at 500 degrees Fahrenheit. This device uses a primary, secondary core and a control core, with the primary winding being simultaneously wound about both the primary and secondary cores, the secondary output winding being wound about the secondary core, and the control core about which the control winding is wound. The control winding and control core are at right angles to the primary core with an air gap existing between the control winding core and the solid primary core. In operation the effect of the magnetic flux in the right angle control core produces a saturation in the primary core whereby the AC produced flux flows proportionally through the secondary core, subsequently inducing a voltage in the secondary output winding.
U.S. Pat. No. 3,123,764 to Henry W. Patton teaches the construction of a magnetic amplifier and control device. The signal is impressed on three windings wound about a plurality of cores with the output being taken from two of the cores with a third core being a nonsaturating member for generating a counter electromotive force in the signal input winding to modify the effects of distributive capacitance currents in the amplifier circuit.
U.S. Pat. No. 3,221,280 to James S. Malsbary et al teaches a saturable reactor which does not require divided reactance or control windings to prevent flow of induced AC of the supply frequency in the control winding and is also used in a polyphase system with a minimum number of separate windings. The patent further teaches a three phase system utilizing the loads being in series with the load windings on the cores and each phase of the power supply around which a single control winding surrounds all three phase cores and a fourth core called an auxiliary magnetic core. In a balanced three phase circuit the algebraic sum of the magnetic flux is equal to zero. If the loads become unbalanced, the flux becomes unbalanced which then produces a current in the control windings. The unbalanced flux produces a current in the auxiliary core which opposes and substantially cancels the current in the control core.
U.S. Pat. No. 3,505,588 to Elwood M. Brock teaches a load impedance responsive feedback system for a variable reactance transformer. The variable transformer has three cores, and primary, secondary, control, and feedback windings. A secondary winding and a feedback supply winding are wound on the secondary winding, while the two auxiliary cores carry DC external control and DC feedback control windings. The primary winding is wound around all three cores.
U.S. Pat. No. 3,343,074 to Elwood M. Brock teaches a variable reactance transformer having two saturable cores. The variable reactance transformer has two saturable cores with control windings, a power core with secondary output winding and a primary winding surrounding all three cores and is wound on top the DC and secondary windings. This device uses control windings wound in series opposition thereby creating a bucking current for any induced voltage in the control windings by the primary current flux. Any residual voltage component is dropped across a shunting resistor in parallel with the two control windings.
U.S. Pat. No. 4,129,820 to Elwood M. Brock teaches a variable reactance transformer having a main core and a pair of auxiliary cores whereby the auxiliary cores carry the DC control windings which are divided in that a first winding is wound about the core and a second coil is wound about the first coil and wherein all the control coils are wound in series and in a configuration such that the induced voltages are substantially zero.
U.S. Pat. No. 4,574,231 to Donald W. Owen teaches a magnetic amplifier apparatus for balancing or limiting voltages or currents. The apparatus comprises of a first level of magnetic amplifiers which are responsive to a DC control signal. The output of the first level magnetic amplifier provides an input signal for a second level of magnetic amplifiers having gate windings to which the alternating current to be controlled is connected.
Although the above stated devices provide control of AC power by means of a DC control signal, all of the devices suffer from a deficiency in that the devices allow an AC voltage to be induced in the DC control windings.
The adverse effects of the induced AC voltage in DC control windings are well known to those skilled in the art. The AC voltages require added considerations to be made in the design and construction of the DC windings and power supplies. Should the AC voltages exist at substantial levels, the counter EMF developed in the DC windings by the AC voltages could not only prevent saturation of the magnetic core of the saturable reactor but severely damage components in the D.C. control circuit. Winding wire sizes and the number of windings become design constraints, and power supplies require large semiconductors or heat sinks to absorb the effects of the AC voltage, adding to unit weight and cost. Elimination of the induced AC voltage allows greater flexibility in both the saturable reactor and associated power supply designs. When no longer constrained by the induced AC voltage the designer may use as many turns as practical in control windings and size the wire to obtain the resistance required for the correct control cur rent. Although attempts to eliminate the undesirable effects of the induced AC voltage in the DC control windings has met with limited success none of the above stated devices has substantially eliminated the unwanted AC voltage. Non-significant differences or variations in cores and windings are sufficient to produce low levels of induced AC voltages in DC windings.
Therefore, it is an object of the present invention to provide an improved variable impedance transformer for controlling the power from an alternating input power source to a load in accordance with a direct current control signal.
Another object of this invention is to provide an improved variable impedance transformer wherein the first and second saturable reactor cores and the first and second power input windings are established and positioned to substantially cancel the magnetic flux proximate the control winding.
Another object of this invention is to provide an improved variable impedance transformer wherein an equalizing winding is simultaneously wound about the first and second saturable reactor cores for shunting any resultant alternating voltage induced by any residual magnetic flux as a result of nonsubstantial physical variations between the first and second saturable reactor cores.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed as being merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention with in the scope of the invention. Accordingly other objects in a full understanding of the invention may be had by referring to the summary of the invention, the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.